1. Field of the Invention
The invention relates to a hard coating excellent in sliding property, formed on respective surfaces of, for example, a die for metal forming, a jig for metal working, and to a method for forming the hard coating.
2. Description of the Related Art
In connection with the die for metal forming, and the jig for metal working, there have since been advanced in improvement of wear resistance and anti-seizing property, effected by nitriding. Improvement of wear resistance and anti-seizing property, to be effected by a vapor phase coating process such as PVD, in place of the nitriding, has lately been under study.
For example, in Patent Document 1, there has been described a technology for improving sliding property by forming a coating composed of a composite nitride containing at least two varieties of metal nitrides among nitrides of Cr, Ti, Al, and V. Further, in Patent Document 2, and Patent Document 3, there has been described a technology for improving sliding property by forming a primary coat comprised of at least one variety of nitride, carbide, and nitriding carbide, containing one variety or more of metallic elements selected from the group consisting of Ti, V, Cr, Al, and Si, as a primary constituent, and forming an uppermost coat comprised primarily of Ti, V, Cr, Al, Si, and Cu or an uppermost coat comprised of a sulfide containing one kind or more of elements among Ti and Cr, and Mo as the balance, on the surface of the primary coat. Still further, in Patent Document 4, there has been described a technology for improving wear resistance and anti-seizing property by forming a surface coat composed of a compound containing molybdenum disulfide, or molybdenum disulfide, as a primary constituent, on the surface of the primary coat of a high-hardness coating comprised of TiN, TiCN, CrN, and so forth.
Furthermore, Patent Document 5 represents the invention for which the present applicant has already submitted the application for patent. In Patent Document 5, there has been disclosed a technology for improving wear resistance and anti-seizing property by forming a coating of composition expressed by (XcM1-c) (BaCbN1-a-b) (where M denotes at least one kind of element among W and V, X indicates at least one kind of element selected from elements of the groups 4A, 5A, and 6A of the periodic table, and the group consisting of elements Al, Si, Fe, Co, and Ni while c, 1-c, a, b, 1-a-b denote atomic ratios of X, M, B, C, and N, respectively).
The coating composed of the composite nitride containing the at least two varieties of the metal nitrides selected among the nitrides of Cr, Ti, Al, and V, as described in Patent Document 1, is a coating high in hardness, and excellent in wear resistance, but the coating is unsatisfactory in anti-seizing property, and is therefore unable to withstand application in harsh environments such as plastic working under a high contact pressure. Further, the coating comprising the primary coat composed of the at least one variety of nitride, carbide, and nitriding carbide, containing one variety or more of the metallic elements selected from the group consisting of Ti, V, Cr, Al, Si, as the primary constituent, and the uppermost coat comprised primarily of Ti, V, Cr, Al, Si, and Cu, formed on the surface of the primary coat, as described in Patent Document 2, is also high in hardness, and excellent in wear resistance as is the case with the coating described in Patent Document 1, but the coating is inferior in anti-seizing property, Still further, the coating described in Patent Documents 3, and 4, respectively, is provided with the uppermost coat composed of the sulfide formed for the purpose of improvement in anti-seizing property. Since the sulfide is relatively soft, this coating is certainly excellent in sliding property at the outset of application, however, the coating undergoes wear over time with the elapse of application time, having therefore a problem in respect of long-term durability.
[Patent Document 1] JP—A No. 2000—144376[Patent Document 2] JP—A No. 2002—307129[Patent Document 3] JP—A No. 2002—307128[Patent Document 4] JP—A No. 2000—1768[Patent Document 5] JP—A No. 2006—124818